1. Field of the Invention
This invention relates to the field of liquid crystal device (LCD) displays, and particularly to grayscale reference generators for such displays.
2. Description of the Related Art
LCD display panels are made up of pixels, with each pixel""s transparency varying with the voltage applied across it. The xe2x80x9csource driverxe2x80x9d circuit which provides the voltages applied to the pixels is typically a simple digital-to-analog converter (DAC). The DAC consists of a series string of resistors with a voltage connected across it such that unique voltages are produced at each resistor-resistor junction, followed by a multiplexer which passes on one of the produced voltages in response to a digital code. As the digital code to the mux is increased from zero-scale to full-scale, the voltage applied across a pixel increases linearly. Unfortunately, the relationship between pixel transparency and applied voltage is non-linear; as such, applying a drive voltage which is xc2xc of full scale may not result in a xc2xc brightness level from an addressed pixel.
Another problem associated with LCD displays is xe2x80x9cghostingxe2x80x9d. Ghosting is avoided by maintaining a near-zero average DC voltage across the pixels. This is accomplished by periodically alternating the polarity of the voltage applied across each pixel. This can be done by, for example, periodically switching one terminal of a pixel between ground and a positive supply voltage, while changing the drive voltages applied to the pixel""s other terminal in synchronization with the switching of the first terminal so as to maintain a constant absolute voltage across the pixel.
Two methods are commonly used to compensate for pixel non-linearity. One method is to correct the non-linearity digitally with the use of a lookup table: the digital code selects an address in a lookup table, which presents a corrected code to the DAC. However, this approach requires what may be an unacceptable amount of computing overhead.
A second approach to correcting the non-linearity involves xe2x80x9cbendingxe2x80x9d the DAC at certain points along the resistor string, using a grayscale reference generator. Several correction voltages are applied to selected resistor-resistor junctions, such that the DAC""s transfer characteristic is made linear between pairs of correction points (though the DAC""s overall transfer function may now be non-linear). When properly arranged, linearly incrementing the digital code to the DAC causes the transparency of a driven pixel to vary linearly. However, because of display non-idealities, when the voltage polarity across a pixel is reversed, different correction voltages may be required to correct for non-linearity.
One method of generating the necessary correction voltages uses on-board resistors to set the correction voltage levels. This requires the use of fabrication masks to select and interconnect the proper resistors, which can be costly and time-consuming. Alternatively, external resistors could be used, but this provides limited accuracy and adversely affects package pin count.
Another method employs a number of DACs equal to twice the number of correction points, with half the DACs providing the necessary correction voltages when the voltage across a pixel has a first polarity, and the remaining DACs providing the necessary correction voltages when the voltage polarity across a pixel is reversed. This scheme requires that each DAC cover the full xe2x80x9cpixel voltage rangexe2x80x9d; as used herein, the xe2x80x9cpixel voltage rangexe2x80x9d is the voltage range required to turn a pixel from fully off to fully on. As such, this approach is inefficient in terms of both power consumption and die area.
A grayscale reference generator is presented which overcomes the problems noted above, using a small number of DACs to provide the necessary correction voltages for an LCD display panel. In one embodiment, each DAC need cover only a limited portion of the full pixel voltage range, thereby increasing accuracy while reducing power consumption and die area.
The present generator is suitable for use with LCD pixels which are responsive over a predetermined pixel voltage range, and which are driven with a source driver having a series string of resistors with connection points for receiving correction voltages. The reference generator includes a plurality of DACS, each of which is arranged to output a respective one of the correction voltages needed to correct pixel non-linearity in response to a digital code word. Each DAC is arranged such that its correction voltage is variable over at least a portion of the predetermined pixel voltage range.
The DACs are preferably driven by a digital multiplexer. The digital mux receives two sets of digital code words: one set which sets the DACs when the polarity across an addressed pixel is positive, and a second set which sets the DACs when the polarity across an addressed pixel is negative. This approach reduces the number of DACs required to produce the correction voltages by half when compared with the prior art method described above.
In a preferred embodiment, the present grayscale reference generator also includes an analog multiplexer connected between the DAC outputs and the resistor string connection points, which is arranged to connect the DAC outputs to the connection points so that the resistor string produces the necessary pixel drive voltages as the polarity of the pixel voltages is periodically reversed. With the analog mux in place, a given DAC""s output can be connected near one end of the resistor string when the polarity of the voltage across a pixel is positive, and then switched to a connection point near the other end of the string when the pixel voltage polarity is reversed. In this way, the number of DACs need only be equal to the number of connection points, yet correction voltages for both pixel voltage polarities are provided. Because each DAC need only cover a limited portion of the pixel voltage range, accuracy is improved and power consumption and die area are reducedxe2x80x94when compared with a scheme using DACs which must cover the full pixel voltage range.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.